This invention relates to a method for producing printed circuits on an insulating substrate by electroless metal plating.
It is well known to apply an electroless metal plating to an insulating substrate, such as plastics, etc. by depositing a noble metal acting as an initiator for electroless metal plating, such as palladium, gold, platinum, etc. onto the surface of the insulating substrate, and then dipping the substrate into an electroless metal plating solution. Two ways are available to form a circuit only on the desired circuit-forming area (which will be hereinafter referred to as "positive pattern") including through-holes on the insulating substrate by the electroless metal plating according to said prior art method.
The one is to selectively deposit the initiator only onto the positive pattern, and the other is to deposit the initiator onto the entire surface of the substrate, and mask other areas than the desired pattern (the other areas will be hereinafter referred to as "negative pattern") by a resist.
Among these two ways, the former ensures a continuous operation from the pretreatment for the electroless metal plating to the electroless metal plating, but is is hard to selectively deposit the initiator onto the positive pattern. In the latter, the resist must be formed on the negative pattern after the deposition of the initiator, and thus the substrate must be dried once on the way as an additional operation. Furthermore, unwanted electroless metal deposition onto the resist on the negative pattern cannot be completely prevented.
U.S. Pat. No. 3,443,988 discloses such an assumption that unwanted electroless metal deposition is caused by surface imperfections such as small dents or scratches which become catalytic by entrapment of electrical charges or activated hydrogen or hydrides during exposure to electroless metal plating solution, and proposes to apply a poison capable of neutralizing catalytic activity in surface imperfections to the resist (column 2, lines 14-26). However, according to tests of the present inventors', it is impossible to deposit the initiator for electroless metal plating only onto the positive pattern.
To selectively deposit the initiator and produce printed circuits by electroless metal plating, for example, the following methods are available:
(1) A method comprising forming a resist film on a negative pattern with a masking material containing an oxidizing agent, and depositing Pd as an initiator on the negative pattern by the oxidation action of the oxidizing agent. PA0 (2) A method comprising forming a resist film on a negative pattern with a hydrophobic masking material containing wax, paraffins, silicone resin or others, and depositing an initiator for electroless metal plating onto a positive pattern. PA0 (3) A method comprising forming a resist on the negative pattern with a masking material containing said oxidizing agent (1), depositing an initiator, removing the initiator only on the resist, and conducting an electroless metal plating only on the positive pattern [German Patent Publication (DAS) No. 2,506,150]. PA0 (4) A method comprising forming a resist having the hydrophobic property 2) on the negative pattern, depositing an initiator, and removing Pd deposited on the resist by means of a compound capable of forming a complex compound with Pd, or a method comprising applying a resist having the hydrophobic property (2) on the negative pattern, depositing Sn.sup..degree.+, removing Sn.sup.++ only on the resist by means of an acid, depositing Pd on the positive pattern in an accelerating step, and conducting the electroless metal plating.
Some effects can be obtained according to these methods. However, it is generally necessary that the film thickness of conductor circuit for the printed circuits be 30-35 .mu.m, and furthermore as to the characteristics of the electroless metal film for the conductor circuit, for example, percent elongation of at least 3% and tensile strength of at least 21 kg/mm.sup.2 are required, as proposed by Institute of Printed Circuits, USA (Specification for electroless copper film for the additive printed wiring application, proposal, Fall Meeting, 1972). Generally, the electroless metal film that satisfies these characteristics can be obtained only under a severe plating condition, at a higher alkaline such as pH 12-13 (20.degree. C.) and a higher temperature of 60.degree.-80.degree. C. Deposition rate of such electroless metal plating is usually about 0.5 - about 5 .mu. m/h, and thus it inevitably takes much time in obtaining the film having a thickness of 30-35 .mu.m.
In said methods (1) and (2), unwanted metal deposition, though not much, takes place on the resist surface 2 to 3 hours after the start of plating, but such metal deposition is not preferable, because it will be a cause for short circuit in the circuits. This means that the oxidizing agent or the hydrophobic action cannot completely prevent the deposition of the initiator such as Pd, etc onto the resist. To improve these disadvantages, the following methods were proposed, for example:
In such methods (3) and (4), it is possible to deposit electroless metal only on the positive pattern, but other problems than in said methods (1) and (2) appear. That is, the oxidizing agent in the resist is weak against acid or alkaline solutions, and thus dissolves in the pretreating solution for the electroless metal plating or the electroless metal plating solution itself, thereby promoting deterioration and decomposition of the solution. For example, PbCrO.sub.4 or PbO has an effect of oxidizing Sn.sup.++ to Sn.sup.++++ and inhibiting deposition of Pd as the initiator. These oxidizing agents dissolve in a highly alkaline metal plating solution, thereby inhibiting the deposition of electroless metal or embrittling the deposited metal films.
On the other hand, the hydrophobic resist containing wax or paraffin cannot withstand the electroless metal plating temperature (60 to 80.degree. C.), and such problems as deformation or melting of the pattern by softening, etc. appear.
U.S. Pat. No. 3,562,038 proposes a method comprising washing merely with an acid without using the resist containing the oxidizing agent or poison. However, when the electroless metal plating is carried out for a long period of time, the electroless metal are deposited even on the resist, and thus it seems that the initiator cannot be completely removed.